I'm not flying jets yet but here's what I remember of my ATPL course...
MCRIT is where the local flow reaches M1.0 over some part of the aircraft. Drag due to shockwaves starts to increase but only becomes significant at MCDR (critical drag rise).
The Operating Manual should give a figure of MRANGE which is the high altitude speed for best range determined by test flights: this MRANGE speed is usually slightly higher than MCRIT.
Going above MCRIT allows you to fly higher for better engine efficiency without the EAS falling below VMD. The increase of engine efficiency and reduction of profile drag at VMD outweighs the slight increase of shockwave drag above MCRIT.
Modern transports operating in the "transonic" regime are by definition in the speed range between MCRIT and MDET.
So I would say cruise speeds are between MCRIT and MMO, although of course some types may vary.